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$4 Hot Air Balloon 



Make] Projects 

hhiiilH ho/ 1 !/ tuMaal/ chare r\icf*f\\tat* 



build, hack, tweak, share, discover,- 



$4 Hot Air Balloon 

Written By: Jesse Brumberger 



PARTS: 



Painter's drop cloth (1) 

Wicker or hard plastic pneumatic tubing (1) 
or similar 

Taped) 

e.g. Scotch tape 

Duct tape (1) 
a few pieces 

Twine (1) 

or similar for the tether line, not susceptible to melting 

Cardboard (1) 

to act as a separator in Step 1 

Stovepipe (1) 

Heat register box (1) 

or heating duct elbow, or scrap sheet metal, for the firebox 

Screen (1) 

or slightly bigger 



SUMMARY 



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$4 Hot Air Balloon 

On a June afternoon in 1978, while most normal kids were focused on sports or getting 
Dad's car keys, I was busy readying another one of my quixotic contraptions, made out of 
junk-at-hand, for its maiden voyage. Assisted by a good friend who also enjoyed such nerdy 
pursuits, I stoked a fire inside an improvised burner and slowly, carefully inflated the large, 
fluttering balloon I had fashioned out of plastic drop cloths, Scotch tape, and coat hangers. 

The sides of the plastic envelope became warm to the touch, and I could feel a slight upward 
tug on the hoop that framed the inlet at the base of the balloon. My friend and I took turns 
steadying the giant transparent chrysalis over the chimney and gleefully fueling the fire with 
old exams and papers from the just-completed school term. Another minute passed; the 
balloon felt very warm and I could feel its positive buoyancy. I wasn't sure how hot the top 
might be getting and didn't want to press my luck. I released the hoop. 

The balloon lifted upward perhaps 10 feet clear of the chimney. It seemed to hesitate there, 
as though comprehending its new freedom, and then accelerated skyward with a swirling 
whoosh. To my total surprise, it kept climbing past treetop level, 100 feet, then 200 feet, and 
began to drift as it climbed. First it cleared the field, then the neighborhood, and then went 
out of sight over the hills. 

Our elation was only slightly dampened by my father's reprimand for our irresponsibility and 
my mother's aggravation over the disappearance of yet more household supplies — aka 
"engineering materials." 

We went on to fly more balloons (tethered), one even carrying a half-pound camera aloft. 
Years later, I bought and flew a ready-made model hot air balloon with my son. While more 
colorful to look at, it didn't fly nearly as well as our homemade versions had. For all you 
readers who enjoy that special kick that comes from seeing an unusual homemade rig 
actually work, here's some fun that can be had on a kite-string budget. 

Specifications 

My original balloons were fashioned from two 9'x12', 0.7mil plastic drop cloths, seamed 
together into a cylinder along their 12' sides. The finished envelope enclosed a volume of 
roughly 230 cubic feet (6.5 cubic meters), weighed 15oz (425g), and would lift again as 
much in payload when thoroughly heated. Unladen, these balloons had lots of extra lift for 
rapid climb and a long flight before cooling enough to descend. 

The single-drop-cloth balloon presented here encloses about 75ft 3 (2.1m 3 ), weighs about 8oz 
(227g), and will provide another 4oz (1 13g) of extra lift when heated to the plastic's safe 

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$4 Hot Air Balloon 



capacity. These smaller balloons provide shorter flights but are much easier to handle. 



Step 1 — Make the balloon envelope. 





• Lay out the drop cloth on a smooth, clean floor. Place a strip of cardboard or wax paper (a 
separator) on top of the drop cloth, running down its centerline, parallel to the 9' sides. 

• Fold the 9' edges over so they meet on top of the center separator. You may find it helpful 
to first place a strip of tape, sticky side up, in the center, then draw the 9' edges to meet at 
the tape so the edges are abutting and parallel. 

• Tape the edges together to form a 9'-long cylinder, trying to overlap the tape evenly onto 
both sides of the seam. Goofed spots can be double-taped later. The separator may now 
be removed. 



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$4 Hot Air Balloon 



Step 2 — Close the top. 




• At one end, gather in the plastic as 
evenly as possible from points 
every V of the way around. Twist 
tightly together for a few inches 
and secure soundly with tape or a 
couple of twist-ties or zip ties. 



Step 3 — Make the inlet. 




• Form the tubing into a hoop about 1 1 /2-2' in diameter and splice it with duct tape. For 
springy materials, reinforce the tape with zip ties. Don't use wire for the hoop (in case the 
balloon runs afoul of power lines), or wooden doweling (it snaps and splinters). 

• Gather the other end of the cylinder as evenly as possible, wrap it around the hoop from 
the outside inward, and tape it every few inches to form a hem a few inches up inside the 
bottom of the balloon. 



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$4 Hot Air Balloon 



Step 4 — Check for holes. 




• Inflate the completed balloon by 
holding the open end in front of a 
small fan. Identify any holes and 
tape them. Tie the tether line to the 
hoop and secure it with tape, or 
use kite line attachment links if you 
wish. 



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$4 Hot Air Balloon 



Step 5 — Pick a heat source. 




• An electric heating gun/paint stripper has enough power to heat the balloon for short test 
hops in the backyard, and it helps inflate the envelope while heating. Other heat sources, 
such as multiple cans of Sterno grouped beneath a short metal chimney, may be 
experimented with. 

• But the classic wastepaper/stovepipe burner provides maximal heating for this balloon. 
You can make the burner from a few feet of 6" stovepipe and a firebox made from an 
aluminum register box, a ductwork elbow, or scrap sheet metal. Black steel or aluminum is 
best, as galvanized steel ducting produces unhealthy zinc-oxide dust when exposed to 
flame. The burner shown here has a firebox fashioned from scrap aluminum flashing and a 
large cookie tin, riveted together in about 10 minutes. 

• Fit the firebox to the bottom of the pipe and place a piece of screen over the top of the 
chimney to prevent burning bits of paper from escaping. 

• Unless you're flying on a steel wire in the middle of nowhere, it should not need saying that 
a heat source should never be sent aloft on a balloon. 



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$4 Hot Air Balloon 



Step 6 — Fly your balloon. 



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$4 Hot Air Balloon 




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• Conditions: Absolutely calm air is 
required for the balloon to be 
manageable while heating. Dawn 
and dusk often provide these 
conditions, as do winter days of 
high pressure when it's clear and 
very cold. You also get more lift 
when it's cold outside, because 
buoyancy is a function of the 
difference in air density outside and 
inside the balloon. 

• Find a location: Identify a flying 
field where you can safely and 
legally make a fire. Bored park 
police may create some real drama 
if they find you building a fire 
outside of a barbecue grill. Keep a 
fire extinguisher or at least a 
bucket of water handy along with a 
pair of leather gloves. As with kite 
flying, make sure no overhead 
power lines are anywhere nearby. 
Be sure to locate your burner on 
sand or stone, off the grass, away 
from flammable scrub, and then 
stabilize it with bricks or a metal 
stake as required. 

• Tether: Do not attempt to launch a 
balloon without a tether. Even when 
it's dead calm on the ground, there 
are always winds aloft, and a well - 
heated free balloon can be lost 
before cooling and descending 
somewhere. As my father angrily 
pointed out to his teenage kid, the 
huge plastic bag could come down 

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$4 Hot Air Balloon 



in traffic, on electrical equipment, 
over a rooftop exhaust pipe, or on 
top of young children. Even if it just 
festoons somebody's treetops, it's 
not a good situation. 

# Prepare the fire: Pay out some 
tether line along the ground before 
launch. Next, prepare a supply of 
newspaper wads several feet away 
from the burner, then start a fire in 
the firebox. 



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$4 Hot Air Balloon 



A 



Step 7 

• Inflate: Now here's the tricky part. Partially inflate the balloon by wafting the hoop through 
the air, and then quickly stand it as upright as possible over the heater while an assistant 
helps. It may be helpful to use a pole to hold up the topknot until the balloon begins to fill 
out. It's critical to keep the plastic from draping too close or touching the chimney, as it 
will instantly melt through. When using the heat gun, keep it moving and keep its end far 
from the plastic. A few small holes near the bottom of the balloon are inevitable and 
inconsequential. 

• The combustion gases flowing up into the center of the balloon are much hotter than 
the airflow near the sides and can sear faces or lungs if you get into their path. Be 
careful not to burn through the tether line! 

• Launch and Ascend: After several seconds the balloon will begin to fill and loft itself. 
Keep the hoop centered over the heat source as you continue heating. It will become 
buoyant quickly. Place a hand against the side of the envelope as high up as you can 
reach. When the side feels very warm about halfway up, perhaps 150°F (65 °C), the top 
will be approaching its limit around 200° F. Check that the tether is secure and release the 
hoop, gently escorting it straight up from the chimney. Pay out the tether line and watch 
your balloon ascend. 

• Experiment: You can experiment with lofting small payloads such as a mini digital 
camera, an altimeter, etc. Try making balloons of different sizes and shapes. Maybe find 
some colored plastic. Or how about a hot-air dirigible with a small electric motor and 
lithium polymer battery? 



This project first appeared in MAKE Volume 29 . page 134. 

This document was last generated on 201 2-1 1 -03 05:23:06 AM. 



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